A conventional loudspeaker disclosed in Japanese Utility Model Laid-Open Publication No. 59-50191 will be described. FIG. 16 is a cross-sectional view of the conventional loudspeaker. FIG. 17 is a cross-sectional view of a diaphragm, an essential portion of the conventional loudspeaker. Magnetic circuit 4 includes upper plate 2, yoke 3, and magnet 1 provided between upper plate 2 and yoke 3. Yoke 3 is coupled to frame 6 with an adhesive. A periphery of frame 6 is adhered to diaphragm 7. Voice coil 8 is adhered to diaphragm 7 and is positioned in magnetic gap 5 of magnetic circuit 4. As shown in FIG. 17, diaphragm 7 has convex portion 7a at the center portion. Convex portion 7a has an arc cross section.
In order to have a small thickness, this loudspeaker includes components having low profiles. If overall height H5 of diaphragm 7 is small, diaphragm 7 has a small rigidity, and may be prevented from transmitting vibration of voice coil 8. This may reduce a sound pressure level in a high frequency range and lower an upper limit frequency, narrowing a reproduced frequency range.
FIG. 18 is a cross-sectional view of another conventional loudspeaker disclosed in Japanese Patent Laid-Open Publication No. 2003-235097. Magnetic circuit 104 includes upper plate 102, yoke 103, and magnet 101 provided between upper plate 102 and yoke 103. Yoke 103 is coupled to frame 106. A periphery of frame 106 is coupled to diaphragm 107 made of a resin film. Voice coil 108 has a circular cross section substantially parallel to diaphragm 107 and has a cylinder shape. One end of voice coil 108 is coupled to diaphragm 107 while the other end of voice coil 108 is positioned in magnetic gap 105 of magnetic circuit 104. An outer shape of diaphragm 107 seen from direction D106 is an elliptical shape that can be placed at a side of a display of a portable telephone. A cross section of diaphragm 107 inside coupling section 107A at which the diaphragm is coupled voice coil 108 has a substantially dome-like shape.
The conventional loudspeaker shown in FIG. 18 has been strongly required to have a small thickness and a small size according to a demand for an electronic device including the loudspeaker, such as a portable telephone, with a small thickness and a small size. In particular, an apparatus receiving stereo sound, such as a portable telephone, includes two loudspeakers provided at both sides of a display, such as a liquid crystal display, thus requiring an area for the loudspeakers that is twice as large as an area for a loudspeaker of a monaural sound apparatus. Furthermore, such stereo sound apparatus includes an electronic circuit for signal-processing and sound-amplifying that requires a larger area than the monaural sound apparatus.
Various types of small, slim loudspeakers have been developed in which their diaphragms do not have circular shapes but have shapes having longitudinal directions, such as ellipse shapes, track-like oval shapes, and rectangular shapes. The slim loudspeakers are positioned at both sides of a display, such as a liquid crystal display, so that its longitudinal direction is parallel to the display. Thus, the shape of the loudspeaker in its plane direction reduces the size of an apparatus for reproducing stereo sound.
The apparatus described above also requires that the loudspeaker has a small thickness. The diaphragm, having a reduced height H106, has a small rigidity, hence causing unnecessary resonance and affecting its sound pressure frequency characteristics.
The slim loudspeakers often include diaphragms having outer shapes other than a circular shape, and a circular voice coil. Thus, a portion of diaphragm 107 from voice coil 108 to outer periphery 107B hardly resonates. Upon being driven by circular voice coil 108, diaphragm 107 having an outer shape other than a circular shape has dispersed resonance frequencies and has an energy not concentrating to a specific frequency since distances from voice coil 108 to outer periphery 107B along diaphragm 107 are different from each other depending on an angle.
The inside of voice coil 108, i.e., an inside of diaphragm 107 from coupling section 107A with voice coil 108 has a circular shape, hence easily resonating. If height H106 of diaphragm 107 is small so as to reduce the thickness of the loudspeaker, diaphragm 107 has a small rigidity, hence often causing unnecessary resonance. Being made of a resin film sheet, diaphragm 107 has a small internal loss and often resonates.
Diaphragm 107 has a cross section with a single arc shape to allow a metal mold for forming the diaphragm to be prepared easily.
The small rigidity of diaphragm 107 provides the loudspeaker with a low upper limit frequency. In order to reduce the thickness of diaphragm 107 having the cross section having the single arc shape, diaphragm 107 has a large apex angle T106 at coupling section 107A, hence reducing the rigidity of coupling section 107A.